The present invention relates to a procees for a cationic cathodic electrocoat by using a cationic electrodepositable paint, and more particularly to a process for carrying out the electrodeposition by supplementing the electrodeposition bath prepared by diluting a cationic electrodepositable paint with deionized water with a supplemental feed paint having a lower neutralization equivalent than the cationic electrodepositable paint in the course of progress of the electrodeposition to control the property values of the above described electrodeposition bath.
The cationic electrodepositable paint coating has been recently practically carried out, because this process can provide a coating having a higher corrosion resistance than an anionic electrodepositable paint coating and obtain a light colored electrodeposition. The paint to be used for the cationic electrodepositable paint coating is based on a film-forming synthetic resin partially neutralized with an organic or inorganic acid, and the electrodeposition bath prepared by using such a paint has a pH value of 3-8.
However, in the bath having a pH value of 3-5, there is no problem in supplementing the paint in the course of progress of the electrodeposition coating, but an electrodeposition tank made of a metal must be insulated at the inner surface with a coating, and the inner surface of installations of pipings and the like in contact with the paint must be formed with an acid resistant material, so that the coating installation cost becomes very expensive. While, when the paint has pH value of 6-8, there has been problems in supplementing the paint to be added to the electrodeposition bath. That is, the process wherein a large amount of surfactant is compounded to a supplemental feed cationic electrodepositable paint having a high solids content to improve the solubility and dispersability of the supplemental feed paint to the electrodeposition bath upon supplementing, has been proposed, but the qualities of the coated film, such as the corrosion resistance and water resistance and the like are lower due to a large amount of the surfactant, and this process has never been practically used.
In addition, the process wherein the supplementing is effected after the neutralization equivalent of the supplemental feed paint is controlled to be substantially equal to the neutralization equivalent of a film-forming synthetic resin contained in the above described cationic electrodepositable paint bath, has been proposed, but in this case, even if the supplementing aptitude is improved, the removal of the excessive organic acid caused during progress of the electrodeposition requires the use of a diaphragm or an ultrafilter. In the former, it is necessary to effect the filtration through a diaphragm to the degree that the solids content in the electrodeposition becomes insufficient, so that the above described solids content becomes insufficient and labor is needed for maintenance of the diaphragm, and therefore this procedure represents a significant detriment to the use of this electrodeposition process in mass production. In the latter, the necessary area of the ultrafilter becomes larger than the case where the ultrafilter is used in the anionic electrodepositable paint coating process, so that the amount of waste water treated increases and the amount of solvent in the electrodeposition bath is reduced. Therefore the cost for the ultrafiltration installation excessively increases, and an improvement has been required.